Endoscopes and borescopes have been available in medical and industrial applications to get images from spaces closed to direct vision. In a multi-fiber endoscope, the image formed at the input end by an optical system is transmitted to the output end in the form of individual pixels by a bundle of fibers, one resolvable pixel per fiber. In other words, relatively large total thickness of endoscope, about 1/8 to 1/4 of an inch is due to large number of the mutually optically isolated fibers. The necessity of the optical system forming the image at the input end of the fiber bundle puts a sufficiently large transverse size requirement on the opening, through which the endoscope is to be inserted into the closed space. Such an endoscope for image transmission through a bundle of individual fibers is shown in U.S. Pat. No. 3,437,747 to E. E. Sheldon and U.S. Pat. No. 5,479,550 by Kimihiko Nishioka et al. to Olympus Optical Co., Ltd, both of which are incorporated by reference.
Furthermore, multi-lens optical endoscopes have a relatively large thickness for the given number of resolvable image pixels, such an endoscope is shown in U.S. Pat. No. 4,036,218 to Olympus Optical Co., which is incorporated by reference.
SELFOC (Self Focussing) and GRIN (Gradient index) waveguides find their use as devices made of one waveguide, where the scrambling of the image by propagation is overcome by the gradient index lensing effect. These endoscopes, apart from having a relatively poor number of resolvable pixels, suffer from the sensitivity to bending of the guide.
Phase-conjugation techniques are known to reconstruct the image which has been blurred by the propagation through an inhomogeneous medium or through a multimode waveguide, by producing "antidistorted" wave and sending the waves back into the medium or waveguide. See for example: B. Ya. Zel'dovich, N. F. Pilipetsky, V. V. Shkunov. Principles of Phase Conjugation, monograph, Springer-Verlag, Berlin, 1985, pp. 1-253; B. Ya. Zel'dovich, V. V. Shkunov. "Optical Phase Conjugation," Scientific American, December 1985, v.253, N6, p.54-59; and D.M. Pepper. "Application of Optical Phase Conjugation," Scientific American, January 1986, v.254, N1, p.37-44. Such experiments were successfully done with the fiber. See for example: G. J. Dunning, R. C. Lind. "Demonstration of image transmission through fibers by optical phase conjugation," Opt. Lett., November 1982, v.7, N11, p.558-560. However the reconstructed image has appeared at the "wrong" end of the fiber which was inside the body in the cases of medical applications. It was suggested to send phase conjugate ("antidistorted") wave into an identical fiber situated outside the closed space. See for example: A. Yariv, J. Appl. Phis. Lett. 1975, v.28, p.88. Such an endoscope is shown in U.S. Pat. No. 5,263,110 to Linvatec Corporation, which is incorporated by reference. Another scheme of an endoscope has been suggested, which uses the optical phase conjugating filter in the middle of the fiber, so that the first and the second pieces of fiber should introduce identical distortion due to propagation. See U.S. Pat. No. 5,263,110 by John E. Anderson, et al. to Linvatec Corporation, which is incorporated by reference.
However, the practical impossibility to create two fibers that would produce identical interference transmission patterns, does not allow the above variants of phase conjugate endoscopy. Furthermore, in the process of the actual use of the fiber endoscope, it is usually subject to bends, and that increases the difficulties of creation of a twin fiber with interferometric accuracy.
The definition of the term "mode" of an optical waveguide can be found in technical literature as a specific solution of wave equation that satisfies the appropriate boundary conditions and has the property that its spatial distribution does not change with propagation, see for example G.P. Agrawal, Wiley-Interscience Pub. N.Y., 1997. We define the term "multimode fiber" or the equivalent term "multimode waveguide" as related to a waveguide that can support propagation of more than one confined mode without considerable attenuation. Multimode optical waveguide(multimode fiber) is different from a fiber bundle both with respect to manufacturing procedure and with respect to the structure.
Fiber bundles are made by parallel arrangement of many individual fibers, typically identical multimode or single-mode fibers. The structure of propagating optical waves in fiber bundles is characterized by very good localization of individual light pixel inside an individual fiber. This is achieved by separation of the cores of the different fibers via claddings that have a smaller refractive index. Multimode waveguides are typically manufactured by longitudinal drawing of a single perform. The localization of the optical waves(modes) in a multimode waveguide is characterized by sharing the space of the optical core by all the modes.
The necessity of much thinner endoscopes, to sustain the number of resolvable pixels being characteristic to existing multimode endoscopes, is governed by the requirement for less invasive techniques for safe penetration through human bones and other tissues, as well as through the walls of containers with hazardous materials.
Several U.S. patents have suggested using the outer shape of the fiber's cladding of a rectangular or square form. See for example: U.S. Pat. Nos. 5,346,655 to Lee L. Blyler, et al. and 5,402,966 to Wolfgang von Hoessle, et al., which are both incorporated by reference. However, the function of this form which is the cladding and not the fiber itself deals with mechanical and geometrical properties of the fibers, and not with the optical properties.
Other U.S. patents deal with the use of waveguides as sensor-transmitters of one signal. See for example: U.S. Pat. Nos. 5,446,279 to Tsung-Yuan Hsu et al. and 4,166,932 to Selway et al., which are incorporated by reference. None of these devices are for image transmission and acquisition.